Field of the Invention
The present invention relates to a novel, practical method of bonding two crystalline silicon bodies without using an adhesive agent.
Theoretically, two solid bodies may be strongly bonded by the chemical attracting force between the atoms or by the cohesive force between the molecules when their surfaces are placed as close to each other as a gap which is equal to the gap between the atoms forming them. Practically, however, such bonding does not occur even if the bodies are placed so close. The reasons may be:
(1) Various gases are adsorbed in the surface regions of the bodies, and the surfaces thereof are in an inert state.
(2) The surfaces of the bodies are uneven. If they are brought into mutual contact, only some atoms of the surface regions contact and the cohesive force between them is insufficient to hold the bodies together.
(3) Usually, scales, dirt, foreign particles, etc., are on the surfaces of the bodies and prevent the direct contact of the bodies.
Hence, in order to bond the bodies without using an adhesive agent, gases should be removed from the surface regions of the bodies, the contact surfaces of the bodies must have a smoothness in the order of the atom size, and the scales, dirt, etc., must be removed from the surfaces of the bodies.
The method most commonly used to bond solid bodies is to use an organic or inorganic bonding agent. During the process, the bonding agent may either be liquid or take a similar state. As long as it remains liquid or in a similar state, it may flow into concaves in the surface of either body and adsorbed into the surface region, expelling the gases therefrom, or it may react with the solid body, thereby firmly bonding the solid bodies.
A method of bonding solid bodies is known which uses no bonding agent and in which two solid bodies of the same material or different materials fuse together. More specifically, the solid bodies are heated, thereby causing creep or fluidizing of their surface regions, whereby the fluidized material fills the concaves in the surfaces, lessens the strain energy at the contacting surfaces, and dissolves and diffuses the foreign matter existing at the interface between the bodies. Hence, the bonding energy at the interface increases.
Neither the method using a bonding agent nor the method of fusing the bodies can be used in some cases. One of these cases is the production of a semiconductor pressure transducer. How this device is manufactured will be described.
As shown in FIG. 1, a semiconductor pressure transducer comprises a single-crystal silicon plate 1, a piezoelectric resistance gauge 2 formed on the upper surface of the plate 1, and a glass substrate 3. The plate 1 consists of a circular thin diaphram portion 1a on which the gauge 2 is formed and a thick ring portion 1b whose lower end is bonded to the upper surface of the substrate 3 by a layer 4 of bonding agent. The glass plate 3 has a central through hole 5. Air pressure 9 is transmitted through this hole 5, thereby deforming the diaphragm portion 1a. As the portion 1a is deformed, the electrical resistance of the gauge 2 changes. This change is detected to determine pressure P.
In the case of the pressure transducer, it is desired that the peizoelectric resistance gauge 2 be very sensitive only to pressure P. In the transducer, however, there is a thermal expansion difference between the plate 1 and substrate 3, which are made of materials having different thermal expansion coefficients. The stress created by this thermal expansion difference acts on the the diaphragm portion 1a, making it impossible for the transducer to correctly detect pressure P. To avoid an inaccurate detection of the pressure, it has been proposed that the substrate be made of a single-crystal silicon, the same material of the plate 1.
Even if the substrate 3 is made of single-crystal silicon, the bonding layer 4, made of Au-Si eutectic alloy, low melting-point solder glass or the like, has a residual stress which also adversely influences the detection of pressure. Accordingly, it is desired that the plate 1 be bonded to the substrate 3 without using a bonding agent.
A method which uses no bonding agent has been invented. This method uses a substrate of borosilicate glass whose thermal expansion coefficient is similar to that of silicon. That surface of the substrate which contacts a single-crystal silicon plate 1 is heated to a glass transition point or a higher temperature. Alternatively, an electric field is applied to the substrate, thus heating the same. When this method is applied in assembling a device which will be used under a high hydrostatic pressure, such as a semiconductor pressure transducer, strain or stress will be created at the interface of the glass plate and substrate since the plate and substrate have different compressibilities.
As generally known, when the surfaces of two glass plates are cleaned and brought into mutual contact, the coefficient of friction between them increases. As a result, the glass plates are bonded to each other. This bonding may be attributed to the fact that the alkali metal ions in either glass plate dissolves into the water-adsorbing surface region of the other glass plate. No silicon body has a water-adsorbing surface region, however. Therefore, it has been thought impossible to bond a single-crystal silicon plate to a glass substrate like a glass plate.